1. Field of the Invention
The present invention relates to a technique of transmitting a realtime digital signal such as a digital audio signal and a music/musical instrument signal, and more particularly to a technique of curtailing the number of terminals in an apparatus for inputting or outputting a digital signal and transmitting the digital audio signal and the music/musical instrument signal in a common format.
2. Description of Related Art
It is being performed generally in recent years that digital audio equipments and electronic musical instruments are connected with one another by digital signal lines, thereby to perform transmission of a digital signal.
A digital audio interface prescribed in a document of IEC 958 for instance (hereinafter referred to simply as a digital audio interface) is used in a digital audio equipment for public welfare or business.
A transmission example of a digital audio signal using a digital audio interface is shown in FIG. 25. In FIG. 25, an output interface 81 is built in a compact disk (CD) player for instance, and an input interface 89 is built in a minidisc (MD) recorder for instance. Further, a digital audio signal written in a register 82 of the output interface 81 is read therefrom and added with an error correction code in a parity bit addition circuit 83, then the signal is subjected to biphase mark modulation and addition processing of a synchronous pattern in a biphase modulation/synchronous pattern addition circuit 84 and is formed into a digital audio signal conforming to the digital audio interface, and is outputted from an output terminal of the CD player for instance. Then, the signal is inputted to the input interface 89 from an input terminal of the MD recorder for instance through a coaxial cable 85 or an optical fiber cable 87. Here, in transmission through the optical fiber cable 87, the digital audio signal conforming to the digital audio interface is converted into an optical signal by means of a transmission module 86 provided in the CD player for instance, and the optical signal is converted into a digital audio signal conforming to the digital audio interface by means of a reception module 88 provided in the MD recorder for instance.
In the input interface 89, the digital audio signal is subjected to detection of a synchronous pattern and biphase mark demodulation processing by a synchronous pattern detection/biphase demodulation circuit 90, subjected to error correction processing in a parity bit check circuit 91, returned to the original digital audio signal through a register 92, and sent to a digital sound recording circuit (not shown) of the MD recorder for instance.
Besides, when digital sound recording is made from an MD recorder to another MD recorder or to a digital audio tape (DAT) recorder, and when the MD recorder is connected to a digital preamplifier provided with a DA converter so as to transmit a digital signal directly, it is required to provide the output interface of the digital audio interface in the MD recorder.
FIG. 26 shows a structure of a subframe of the digital audio interface. Further, FIG. 27 shows the structure of subframes, frames and blocks of the digital audio interface.
As shown in FIG. 26, in a protocol of the digital audio interface, the subframe transmits a channel 1 (left channel) or a channel 2 (right channel) of a stereo signal and channels 1, 2, 3 and 4 of a four channel stereo signal. Further, the subframe is composed of 32 bit portions in total, as follows.
1 Sync. preamble . . . 4 bit portions from b0 to b3
2 AUX (auxiliary bit) . . . 4 bit portions from b4 to b7
3 Audio data . . . 20 bit portions from b8 to b27
4 Validity flag . . . 1 bit portion of b28
5 User data . . . 1 bit portion of b29
6 Channel status . . . 1 bit portion of b30
7 Parity bit . . . 1 bit portion of b31
As shown in FIG. 27, the frame attains a length of 64 bit portions which is twice as long as the subframe. In the CD, the sampling frequency is 44.1 kHz, and a 2-channel stereo signal having 16 bits are recorded. When a CD signal is transmitted by the digital audio interface, the MSB of the CD data in 16 bits is placed in b27 of the subframe of the digital audio interface, and up to the LSB are placed thereafter in b12. Further, 0.sub.2 is placed in 4 bits of b11 to b8 of the subframe and b7 to b4 of the AUX. Thus, a transmission velocity in the digital audio interface of the CD signal becomes 44.1 kHz.times.64 bits=2.8224 Mbps. Further, in the digital audio interface, the sampling frequency corresponds to 48 kHz and 32 kHz other than 44.1 kHz.
In channel coding of the digital audio interface, biphase mark modulation in which logic "0" is expressed with two bits 00.sub.2 or 11.sub.2 having a period of T/2 when it is assumed that the bit period is T and logic "1" is expressed with 01.sub.2 or 10.sub.2 of T/2 is performed. The maximum inversion spacing of biphase mark modulation is the bit period T, and the minimum inversion spacing thereof is T/2.
A unique symbol including 3T/2 which is against the rule of biphase mark modulation is used for synchronization and preamble. This symbol includes three types, i.e., the start of the block and the start of channel 1 B, the start M of the channel 1, and the start W of the channels 2, 3 or 4, and
B: 11101000.sub.2 or 00010111.sub.2
M: 11100010.sub.2 or 00011101.sub.2
W: 11100100.sub.2 or 00011011.sub.2 are used.
As shown in FIG. 27, one block is composed of 192 pieces of frames, and the preamble of B is placed at the start of the block. Further, it is possible to form a channel status in a table of 192 bits of one block portion, thereby to transmit various data. Besides, no data corresponding to a control signal of the equipment and an address of the equipment are prescribed in this table.
Since no address information is added to the digital audio interface, point-to-point communication, i.e., data transmission only among apparatuses connected with cables is performed. Accordingly, in an apparatus that becomes the center of signal connection such as a television set (TV) of video apparatus and an amplifier and a receiver of an audio system, signal lines of digital audio interface are connected concentratedly in a tree shape from a plurality of digital audio equipments.
FIG. 28 shows an example of a system in which a plurality of audio equipments and video equipments are connected concentratedly to a digital amplifier. In this example, a digital sound program tuner 101, a speaker 103, CD players 104 and 105, an MD recorder 106, a DAT recorder 107 and a digital video cassette recorder (hereinafter referred to as a DVCR) 108 are connected concentratedly to a digital amplifier 102 provided with a DA converter.
Further, respective apparatuses are connected to one another unidirectionally or bidirectionally by means of signal lines (coaxial cables and optical fibers) of the digital audio interface described previously. Since the digital audio interface is capable of unidirectional transmission only, two signal lines are provided among apparatuses connected bidirectionally (among respective apparatuses of the MD recorder 106, the DAT recorder 107, the DVCR 108 and the digital amplifier 102).
In the system shown in FIG. 28, in order to make sound recording or the like in the MD recorder 106 from the CD player 104 or 105 for instance without assistance or automatically, it is necessary to transmit control signals for the above among these apparatuses. However, since no method of transmitting such control signals is prescribed in the digital audio interface as described previously, it is necessary to jointly use another interface for control. Therefore, the digital amplifier 102 is connected to respective apparatuses by control buses. There are various standards in such an interface for control.
Further, in recent years, performance information, control information, synchronizing information or the like are transmitted among electronic musical instruments connected with one another by the interface prescribed by Musical Instrument Digital Interface (MIDI) standards (hereinafter referred to simply as MIDI musical instruments).
The MIDI standards include three types of terminals such as a MIDI IN terminal (hereinafter referred to as IN), a MIDI OUT terminal (hereinafter referred to as OUT) and a MIDI THRU terminal (hereinafter referred to as THRU), and MIDI musical instruments are equipped normally with IN and OUT and THRU is also provided in many musical instruments. THRU is a terminal having a function of outputting the MIDI signal inputted from IN as it is.
When OUT of a MIDI musical instrument (master) 110 is connected to IN of a MIDI musical instrument (slave) 111 with a MIDI dedicated cable (hereinafter referred to as a MIDI cable) and a keyboard of the MIDI musical instrument 110 is played, the MIDI musical instrument 111 is performed together. That which becomes the master is an equipment generating data of MIDI such as an electronic musical instrument and a sequencer having a keyboard and is called a MIDI controller. As the slave, a sound source module of only a sound source having no keyboard, an effecter and all of other MIDI musical instruments can be connected.
It is possible to connect OUT of a MIDI musical instrument 112 to IN of another MIDI musical instrument 113 and further connect THRU to IN of another MIDI musical instrument 114 in a cascade manner, and furthermore, to connect THRU to IN of another MIDI musical instrument 115 in a cascade manner as shown in FIG. 30, thereby to play a plurality of MIDI musical instruments at the same time. Since the MIDI signal is deteriorated every time it passes through the THRU, however, three sets to four sets are normally the limit in cascade connection by means of THRU such as shown in FIG. 30.
Therefore, in order to connect a plurality of MIDI musical instruments to one another, a method that the OUT of a MIDI musical instrument 116 is inputted to a parabox (referred to also as a THRU box) 117 and the outputs of the THRU box are connected to IN of a plurality of MIDI musical instruments 118 to 121 as shown in FIG. 31 is used. When a large number of MIDI musical instruments are connected, however, there is such a problem that MIDI cables are concentrated upon the THRU box and so on.
In normal MIDI communication, transmission in an open loop is performed having no concern on the transmitting (master) side in whether the receiving (slave) side is receiving correctly or not. However, when the data quantity is large among MIDI signals as the transmission of sampling data described later, the data are split so as to perform packet transmission. At this time, there is provided a function that an error check for confirming whether the data have been sent correctly or not is made, and retransmission is demanded in case the data have not been sent correctly. This is called transfer by handshake. When handshake is made, it is required to connect IN of a MIDI master 122 to OUT of a MIDI slave 123 with the MIDI cable as shown in FIG. 32.
The MIDI musical instrument is provided with terminals IN and OUT, but unidirectional communication in which standpoints of the master and the slave are fixed is performed and no correspondence to bidirectional communication is made. Therefore, there is such a problem that, even when the keyboard of the MIDI musical instrument which has been set to the slave once is played, the MIDI musical instrument of the master cannot be sounded. Further, in a cascade-connected system shown in FIG. 30, and in a system using the THRU box shown in FIG. 31, the equipment which is used as the master is determined in advance, and it is required to determine the order of connection of the MIDI musical instruments for performance. As a result, resetting when the musical instrument is moved is inconvenient, thus causing such a problem that the structure of a musical instrument that is set once is difficult to be altered and so on.
A message transmitted among MIDI musical instruments is called a MIDI message. The MIDI message is expressed by a byte train in one byte or more. As shown in FIG. 33, the byte train of the MIDI message is divided into a status byte and data bytes. The status byte expresses the type of the MIDI message, and the MSB bit 7 is "1". The status byte is normally accompanied by data bytes in a definite number. However, some messages are not accompanied by data byte. The MSB bit 7 of the data byte is "0".
As shown in FIG. 34, the MIDI message is classified into two, a channel message and a system message. The channel message is performance information for controlling individual musical instrument, and the system message is control information, synchronizing information or the like for controlling the whole MIDI system. Since a control command related to connection among musical instruments and so on is not allocated to the MIDI message, setting of the system, alteration of the structure or the like cannot be carried out through MIDI standards.
The system message is classified into a system exclusive message, a system common message and a system realtime message. The channel message includes a channel voice message (hereinafter referred to as a voice message) and a channel mode message (hereinafter referred to as a mode message).
In the MIDI message, the number of data bytes becomes two bytes or less excluding the system exclusive message, i.e., three bytes or less including the status byte. In FIG. 34, a small letter n expressing the status byte with a hexadecimal number is used for designating the MIDI channel.
A method of designating the MIDI channel is shown in FIG. 35. When a plurality of MIDI musical instruments are connected to a MIDI controller by MIDI cables in FIG. 30 or FIG. 31, it is possible to perform individual MIDI musical instrument independently by designating the MIDI channel. The MIDI channel has four bits and is able to designate 16 channels at the maximum.
FIG. 36 shows an example of a method of using the MIDI channel. To a MIDI controller 124 are connected three sets of MIDI musical instruments 125, 126 and 127. The MIDI musical instrument 125 sets tones of a saxophone, the MIDI musical instrument 126 sets tones of a piano, and the MIDI musical instrument 127 sets tones of an electric base. Further, when performance data for respective parts are sent from the MIDI controller 124, respective musical instruments generate sound with respect to each part.
As described, it is possible in the MIDI standards to designate a specific MIDI musical instrument among a plurality of MIDI musical instruments on the receiving side (slave) and transmit a message by the MIDI channel. However, since four bits in lower positions of the status byte are used for designation of the MIDI channel, it is difficult to increase the MIDI channel to a number of 16 or more.
As shown in FIG. 33, the operation is performed with asynchronous serial transfer at a transfer speed of 31.25 kbps (.+-.1%) of the MIDI standards. The transfer is made in the order of start bit, bit 0, . . . , bit 7 and stop bit (10 bits in total), and the start bit is a logic "0" and the stop bit is a logic "1". 10.times.(1/31.25 kHz)=320 .mu.s is required for transferring one byte. Since three bytes are required for a message of note-on that produces one sound in the MIDI system, it takes 320 .mu.s.times.3 =approximately 1 ms for producing one sound by MIDI.
A MIDI musical instrument provided with a sampling function is called a sampler. Sampling means that a user produces live tones of musical instruments in the form of digital data and records these data in a memory or the like. Further, at time of reproduction, the digital data are taken out of the memory at free timing so as to produce a sound. Sample dump which is one of the universal system exclusive messages is used for the transmission of the sampling data taken out of the sampler. With the universal system exclusive message, it is possible to transmit/receive data among MIDI musical instruments manufactured by different manufacturers. The sample dump is a common format for transmitting the sampling data of the sampler.
FIG. 37 shows data formats of three MIDI messages, sample dump request, dump header and data packet. When dump request in (1) is outputted with a MIDI musical instrument, dump header in (2) is transmitted and then data packet in (3) is transmitted. The data packet has a fixed length of 127 bytes, and the data length is 120 bytes at the maximum.
Normally, the sampled waveform data include several ten Kbytes, and it is required to transmit a large number of data packets. When such a large quantity of data are transmitted at a time, a lot of time is required for processing of transmission and reception of MIDI, thus making it impossible to transmit performance information such as a channel message at the same time. Therefore, a switch for turning off so as to receive no system exclusive message is normally provided in the MIDI input/output function.
As described above, there have been problems set forth in (1) and (2) below in conventional digital audio interface standards.
(1) Two transmission lines become necessary in order to conduct bidirectional transmission, and two terminals of input and output are required on the apparatus side. PA1 (2) The transmission lines are increased in an apparatus that becomes a center of the system, and a plurality of terminals of input and output are concentrated. When another interface is required for control, the number of terminals for inputting to and outputting from the apparatus is increased. PA1 (3) Notwithstanding that IN and OUT terminals are provided in the MIDI musical instruments, unidirectional communication only is prescribed, thus being unadaptable to bidirectional communication. PA1 (4) The MIDI musical instrument which becomes the master of the system is fixed, thus making it impossible to construct a flexible MIDI system. PA1 (5) When a plurality of musical instruments are connected, cables are concentrated to the THRU box. PA1 (6) The transmission speed being slow, it is difficult to transmit a large among of data. PA1 (7) Connection information of the system and control commands for connection are not prepared.
Further, there have been problems set forth in (3) to (7) below in transmission of a message according to conventional MIDI standards.
Furthermore, it is expected to execute multiplex synthesis of performance information of the electronic musical instrument on a digital audio signal reproduced by a digital audio equipment using the digital audio interface described previously such as a CD player, or to execute multiplex synthesis of performance information of the electronic musical instrument on a voice (vocal) recorded/reproduced by an MD recorder and record it digitally, but it has been difficult to connect the electronic musical instrument and the digital audio equipment digitally because data formats and data transmission speeds of the MIDI standards and the digital audio interface are different from each other.